1. Field of the Invention
The present invention relates to the production of flame retardant agents and more particularly to a novel process for producing certain polybrominated higher alkyl benzenes useful in flame retarding various thermoplastic resin systems.
2. Description of the Prior Art
Traditionally, most flame retardants, although efficient in their function of retarding the rate of combustion in a resin system, have a tendency to affect adversely one or more key properties of the resin. For example, many flame retardant additives tend to reduce impact strength of the resin; to migrate from the resin composition, resulting in a phenomena known as "bloomp"; to volatilize from the resin composition; to plasticize the resin composition adversely, thus lowering the heat deflection temperature, etc.
It is therefore essential that flame retardant agents be specifically tailored to the resin system so that, in addition to its role as a flame retardant, the agent will additionally enhance the desirable characteristics of the resin composition. Those skilled in the art well known that the selection of such an application-specific flame retardant is unpredictable at best. Thus, even though a given agent may exhibit utility in a particular resin system, that is no guarantee that this agent will have any uses at all with other resins.
It has been discovered, and is the subject of certain other copending patent applications filed herewith, namely, Ser. Nos. 107,228; 107,236; 107,627; and 107,700, all filed Oct. 9, 1987, that, quite unexpectedly, certain brominated higher alkylbenzenes are capable of functioning in a highly satisfactory manner in a number of unrelated resin systems. It has been generally observed that a high loading of many additive-type flame retardants produces a detrimental effect on the physical properties of the resin. Therefore, the accepted procedure has been to use an additive with high bromine content, thus minimizing its weight content in the resin and consequently reducing is deliterious impact on the resin. Brominated compounds with less than 65% bromine are generally considered of marginal or of no interest because, in order to impart flame retardancy to the resin (say 10 weight percent bromine), at least 16 percent by weight of the additive must be added. In many resins, such high loading of the additive significantly deteriorates the physical properties of the resins. However, in the resin systems to which the present invention has application, the alkyl substituent in the benzene ring imparts desirable properties which compensate for its high loading, especially in ABS. However, the art has not taught a satisfactory process by which such brominated higher alkyl benzenes can be produced.
More particularly, no satisfactory bromination technique exists for the prparation of polybrominated higher alkylbenzenes (alkyl.gtoreq.C.sub.6), especially mixtures of alkylbenzenes containing high concentrations of secondary alkyl groups. Hennion and Anderson (J. Am. Chem. Soc. 68, 424 [1946]) studied the bromination of a wide variety of alkylbenzenes in liquid bromine medium and a small amount of aluminum catalyst. The authors found that in all cases secondary and tertiary alkyl groups were replaced by bromine. However, methyl and ethyl groups were left intact. Replacing aluminum catalyst with a less vigorous iron powder catalyst led substantially to the same results. Thus, bromination of n-propylbenzene led to pentabromo-n-propylbenzene, while isopropylbenzene yielded hexabromobenzene. Additionally, bromination of sec-amylbenzene; sec-octylbenzene, p-diisopropylbenzene all led to hexabromobenzene as product. The authors concluded that only primary alkyl groups survived the bromination.
Mills and Schneider (Ind. Eng. Chem., Prod. Res. Dev 12 (3), 160 [1973]) described the reaction of bromine chloride with aromatic compounds. They showed that benzene could be successfully brominated by BrCl in chlorinated solvents using ferric chloride and aluminum chloride catalysts. Likewise ethylbenzene was brominated by BrCl to 4-bromoethylbenzene. The authors did not show any examples of polybrominated alkylbenzenes, however. In another article, Lamneck Jr., (J. Am. Chem. Soc. 76, 1106 [1954]) described the preparation of monobromo derivatives of propyl- , isopropyl- , butyl- , isobutyl- and sec-butylbenzenes. The bromination was carried out in acetic acid with no catalyst. However, the described bromination produced only monobrominated alkylbenzenes at relatively poor yield.
Barda, et al. U.S. Pat. No. 4,352,909 disclosed that polystyrene can be brominated to the tribromo level by BrCl in a chlorinated hydrocarbon solvent in the presence of a catalytic amount of a Lewis acid, specifically antimony trichloride. While the patentees teach that tribrominated polystyrene may be obtained under the conditions described, higher levels of nuclear bromination do not appear attainable using the Barda, et al. process.
Underwood, et al. U.S. Pat. No. 3,850,882 discloses a three component flame retardant additive system for polyolefins, especially polypropylene, consisting of
(a) among other halogenated materials, a halogenated alkyl benzene of the formula: ##STR1## where X may be Cl or Br; and Y is a hydrocarbon of 1-20 carbon atoms; a is an integer from 0 to 3; and n is an integer from 3 to 6.
(b) Stannic oxide;
(c) a bis-phenylalkylene hydrocarbon.
The patent does not disclose the synthesis of brominated alkylbenzenes, especially mixtures thereof. Nor does the patent specify whether the included bromoalkylbenzenes are primary, secondary, or tertiary.
Rueter, et al. U.S. Pat. No. 4,129,551 disclosed nonflammable polyester compositions incorporating a phosphorus-containing, multiple component flame retardant additive consisting of:
(a) a triarylphosphine oxide or an aryl or alkyl ester of an arylphosphinic acid;
(b) a nuclear brominated alkylbenzene; and
(c) customary auxiliary agents and additives.
Polyester compositions based on such agents contain 0.5-10% by weight of bromine and 0.1-2% by weight of phosphorus. Among the nuclear brominated alkylbenzenes described were compounds of the following formula: ##STR2## where x =2 to 5; y and z each are zero or an integer from 1 to 17; and the sum of y+z is an integer between 7 and 17. Mixtures of such compounds are also disclosed. No disclosure is made of the synthesis of brominated alkylbenzenes although the authors suggest that nuclear brominated compounds may be made by known methods as described in the above-described Hennion, et al. and Mills, et al. papers. However, as noted, products produced by these methods yield only brominated primary alkyl benzenes.
Thus, none of the prior art describes a technique for successfully polybrominating higher secondary or tertiary alkylbenzenes or mixtures thereof.
A primary objective of this invention is to provide methods of synthesis of highly brominated higher alkylbenzenes from readily commercially available raw materials.
A related object is to provide methods of the character described that are especially useful in producing polybrominated higher alkylbenzenes.
A further object is to provide methods of the character described useful in producing polybrominated mixtures of secondary and/or tertiary alkylbenzenes.
A still further object is to provide methods for producing mixed liquid polybrominated secondary alkylbenzenes.
Yet a further object is to provide a method for brominating higher alkylbenzenes from the corresponding hydrocarbon materials in excess bromine as a reaction medium utilizing bromine chlroide as the brominating agent and an antimony halide as catalyst.